Solid State Drives(SSD)-Things to Know

Solid State Drives (SSD) are all higher performance than traditional spinning hard drives, but organic…there are a maximum number writes and the products will fail. For the last five years, Samsung has established that they are a market leader in SSD technology, Intel is number two because they are more expensive, but still of high quality. There are a lot of manufacturers trying to enter the market, which you will find in a quick search. There are different grades of SSD drives. Think: consumer, business, enterprise grades.

eMLC (Enterprise multi-level cell, 2 bit=more right cycles, relatively new in the last couple of years) drives with their enhanced garbage collection techniques have closed that gap between MLC and SLC but only by a small amount.

MLC (multi-level cell, original was 2 bit, but now more likely 3 or 4 bit, and fewer right cycles) hybrid drives. These were actually first introduced back in 2008 by Silicon Motion. They were the ones who first produced the SSD controllers which enabled a hybrid SLC-MLC NAND drive.

TLC (Triple Level Cell) would be like that found in USB (4 bit or higher, lower quality and cheaper, runs the hottest). (For example, Samsung Evo is TLC NAND)

The fact remains that SLC SSD’s (enterprise) are higher endurance drives with regards to writes.

Life of drives

An SLC drive is rated at 100,000 writes per cell expected lifetime. They are the most expensive (you can buy from HP, 300GB=$8,000)

An eMLC drive can reach numbers of 20,000 – 30,000 writes per cell expected lifetime. This is the sweet spot for enterprise. These are being used in data centers. An example is the Samsung sm843 SSD eMLC Enterprise cited below.

An MLC drive is 10,000 writes per cell expected lifetime.

Consumer MLC = 3,000-10,000 writes

From those numbers and with the current cost of eMLC drives I believe the cost vs. writes is a no brainer. These drives when placed in a RAID array should provide a relatively nice service life at a fraction of the cost of SLC drives.

Performance in Servers and SANs

There is not as much gap in server and SAN controller performance as there once was. Many drives sit behind a caching engine as Tier1 space. The cache (Tier0) handles the processing and write optimization (more on this further below).

Latency is not a concern with today’s drives. As with the above, these drives also have the luxury benefit of sitting downstream of the Tier0 cache controller as the second tier. The cache is 30X faster than SSD on write operations and keeps “hot data/pre-fetch” for faster reads. Out-of-cache reads (Achilles’ heel of caching) are done at SSD speeds. SSD drives are super-fast on reads. This method prevents “write cliffs” that some SSD engines experience as the drives fill up with writes.

Endurance numbers are much, much, much higher than 2011. With the right choice of drive, the expected lifetime is 19 years under enterprise class conditions. Drives in an “abusive” environment should last five years. We recommend changing the drives by then in those environments (drives will be larger/faster with even higher endurance numbers in the future). Firmware frequently monitors the SMART data on SSD for replacement prediction based on utilization.

Reservation space is large enough on most drives to handle multiple years now (used when a cell reaches lifetime use).

Products and product review

Samsung just released the 850 PRO which is the successor to the 840 Pro that is mentioned below. It is too early yet to know on this device although it looks to be on track to be just as successful as the 840 Pro. The 850 Pro would fall into the Performance category just like its predecessor the 840 Pro.